1. Field of the Invention
The present invention relates to an imaging unit and electronic camera provided with a correction mechanism containing two drive devices for independent driving in two directions intersecting at right angles to correct the wobble (also called the camera shake) of the two-dimensional image formed on the image forming surface of an image pickup element.
2. Description of Related Art
The wobbles in various directions occurring in an electronic camera that give the most serious impact on the image wobble on an imaging screen are the image shift wobbles (translational motions of an image on the plane surface perpendicular to the imaging optical axis, i.e., on the image forming surface) produced by the angle wobble (inclination of imaging optical axis) of the optical axis of the imaging optical system. These angle wobbles are detected after being decomposed into the angle wobble in the vertical direction (pitch direction) and that in the lateral direction (yaw direction) for the sake of expediency. Based on the detection signal, driving is provided in the vertical and lateral directions independently on the plane surface perpendicular to the optical axis of the image pickup element with respect to the imaging optical system, whereby the two-dimensional wobbles on the image forming surface can be corrected (Japanese Unexamined Patent Application Publications Nos. 2006-174226 and 2007-206553).
The imaging unit disclosed in the Japanese Unexamined Patent Application Publication No. 2006-174226 is provided with a Y-stage movable along the guide rod Y mounted on a fixed holder, and an X-stage movable along the guide rod X carrying a CCD as an image pickup element and mounted on the Y-stage. To correct camera shake, the Y-stage is driven by a Y-direction drive device and the X-stage is driven by an X-direction drive device.
To ensure smooth relative motion, some clearance is provided between the guide rod Y and Y-stage, and between the guide rod X and X-stage. Camera shake can be corrected by a CCD of relatively low pixel without any particular problems. However, an increase in the pixel level of the image pickup element is promoted to get a high-quality image, whereas a more compact configuration and lighter weight of the camera body are required at the same time. In this context, the image pickup elements of smaller size and higher-level pixel are utilized particularly in recent years, and therefore, the pixel pitch tends to be reduced. Further, if the image pickup element of a smaller pixel pitch is used, wobbles resulting from the clearance between the guide rod and stage cannot be ignored. There is concern about the possibility of failing to ensure high-precision camera shake correction. In the meantime, if the clearance is set at a smaller value, restriction is imposed on smooth movement, and a new problem arises. Another problem is that such a clearance will be increased by chronological change due to friction and others.
Further, in the structure disclosed in the Japanese Unexamined Patent Application Publication No. 2006-174226, the Y-direction drive device includes a motor fixed to a fixed holder, a feed screw driven by the motor, and a nut connected on the Y-stage and changing the rotating motion of the feed screw into the motion in the axial direction. The X-direction drive device includes a motor mounted on the Y-stage, a feed screw driven by the motor, and a nut connected to the X-stage and converting the rotating motion of the feed screw into the motion in the axial direction. However, to achieve high-precision driving, it is preferred that the nut and stage should be biased in one direction at all times to remove wobbles between the nut and feed screw. In the meantime, to absorb the impact in the event of accidental falling, elastic connection between the nut and stage is preferred. Thus, in Patent Document 1, two springs for biasing the nut and stage are provided in each of two directions. However, arrangement of a total of four springs increases the dimensions in the X- and Y-directions. This will raise the problem of increased size and heavier weight.
To meet the requirements for cost reduction, attempts have been made to use a less costly stepping motor as the drive source of the camera shake correction imaging unit. However, since the stepping motor inherently has no positioning sensor, uneven rotation is caused by greater loads, with the result that positioning accuracy is deteriorated. To solve this problem, it is necessary to take such steps as increasing the clearance of the guide section in order to minimize the load of the member to be driven by the stepping motor. However, increasing clearance of the guide section causes mechanical play or looseness during movement. This causes unstable control of the camera shake correction and reduces the image quality—a new problem in this effort.